1. Field of the Invention
The present invention relates to a cell evaluating method and a cell evaluating apparatus suitable for use in evaluation of a secondary cell of various kinds such as a lithium ion cell or the like.
2. Description of the Related Art
In the prior art, as the secondary cell a nickel cadmium cell has been widely used, but cells such as the lithium ion cell, a nickel hydrogen cell and so on are newly developed as a cell of high performance and now improvement of those cells will be conducted. In this case, when those secondary cells are developed or improved, there is a need to evaluate characteristics of the cell gained by connecting the cell to the evaluating apparatus.
Further, in a case of the secondary cell the characteristics of which are already known, there is a need to know what state the cell is in as a result of the cell having been used. For example, when a cell is repeatedly used a plurality of times, there is a case in which its capacity capable of being charged decreases. In such a case, in order to know to what extent the capacity of the cell has decreased, it is necessary to evaluate the characteristic such as the chargeable capacity or the like by connecting the cell to the evaluating apparatus.
In a conventional cell evaluating apparatus, the evaluation of the characteristics of the connected secondary cell is made from changes in the cell voltage and the charge and discharge current by practically conducting charging and discharging of the cell, but there is a case that the characteristics of the cell can not be correctly evaluated by simply conducting charging and discharging of the cell.
That is, when a charging is started, for example, there is a need to judge whether or not a cell connected for the evaluation is a so-called short-circuited cell in which the positive and negative electrode sides thereof are short-circuited. It has been customary to judge as the short-circuited cell a cell whose voltage doesn't rise to a reference voltage soon after the charging is started. However, in fact, even a cell judged as the short-circuited cell by this process, there is a cell which can be restored to a normal cell depending on a process since its cell voltage abnormally is lowered due to an excessive discharging or the like. It can not be said that the cell is correctly evaluated.
Moreover, it has been difficult to detect a phenomenon called a micro-short circuit, which is one of the inner-short-circuit states within the above cell, by the conventional evaluating apparatus. That is, a secondary cell becomes a cell in a state of being micro-short circuited because of an intrusion of small extraneous stuff and an occurrence of tree-branch like crystals called a dendrite. With the micro-short circuited cell, a cell voltage once rises to a voltage corresponding to a full charge voltage by an ordinary charge, but if the cell is left as it is, its voltage drops faster than the lowering of a voltage caused by a self-discharge. The lowering of the cell voltage, however, is a phenomenon that takes several days even if it is said that the voltage lowering is faster than the self-discharge. So it has been impossible to detect the micro-short circuited cell by connecting the cell to the conventional evaluating apparatus for merely two or three hours.
In a case of a secondary cell like the lithium ion cell, if the cell is charged to its full charge, a process is carried out by an ordinary charger in which a cell voltage and a charging current are monitored, and when a predetermined voltage and a predetermined current are respectively reached, it is judged that the cell is fully charged, and the charge is stopped. However, it is not necessarily said that the cell is controlled to a correct state of being fully charged though the cell is nearly fully charged by simply judging and controlling the voltage as well as the electric current at a time of charging due to the above reason. So in order for the evaluating apparatus to make the correct evaluation of the characteristics of the cell, the more precise process is demanded.